The use of a laser beam for scanning an original and/or recording information on a laser sensitive medium has been disclosed in the prior art as shown, for example, in U.S. Pat. No. 3,946,150. For recording purposes, a laser beam is directed to a modulator which modulates the laser beam in accordance with information to be printed on the laser sensitive medium, such as a charged xerographic surface. The modulator beam is directed to a rotating optical scanner, driven by a motor, which acts to scan the beam across the xerographic surface.
While for relatively low-accuracy requirements the motor driving the rotating scanner can be made to function adequately without speed sensing and feedback control, this generally can only be accomplished by careful and perhaps expensive attention to the elimination of all sources of disturbances of the motor.
It has been found that for accurate speed control at relatively high speeds a hysteresis synchronous motor would be preferable to drive the rotating scanner. However, these motors tend to exhibit characteristic speed variations called "hunting". Hunting is inherent in systems using hysteresis synchronous motors because of time lags in the response caused principally by energy storage in the circuit inductances, especially in the field-winding inductances. Thus, a sudden change in the motor shaft load causes a deviation in the speed from its set value and correction of the speed cannot take place without the lapse of a definite time to permit readjustment of the stored energies. This time lag between disturbance and response may result in successive over-shooting and under-shooting of the correct speed, representing serious oscillations about the final speed value.
The prior art discloses various systems for controlling the speed at which a synchronous motor operates, such as those disclosed, for example, in U.S. Pat. Nos. 3,144,595 and 3,723,908. However, the prior art systems do not provide the precise speed control required in high resolution and close registration tolerance optical scanning systems.